Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies, 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support for the next decade. The ability to provide high bit rates is a key measure for LTE.
The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is finalized as Release 8 (LTE Rel. 8). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP) and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmit power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques and a highly efficient control signaling structure is achieved in LTE Rel. 8/9.
LTE Architecture
The overall architecture is shown in FIG. 1 and a more detailed representation of the E-UTRAN architecture is given in FIG. 2. The E-UTRAN consists of an eNodeB, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC) and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming user equipments.
Further Advancements for LTE (LTE-A)
The frequency spectrum for IMT-Advanced was decided at the World Radiocommunication Conference 2007 (WRC-07). Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved. The study item covers technology components to be considered for the evolution of E-UTRA, e.g. to fulfill the requirements on IMT-Advanced. Two major technology components are described in the following.
Carrier Aggregation in LTE-A for Support of Wider Bandwidth
In carrier aggregation, two or more component carriers (component carriers) are aggregated in order to support wider transmission bandwidths up to 100 MHz. Several cells in the LTE system are aggregated into one wider channel in the LTE-Advanced system which is wide enough for 100 MHz even though these cells in LTE are in different frequency bands.
All component carriers can be configured to be LTE Rel. 8/9 compatible, at least when the aggregated numbers of component carriers in the uplink and the downlink are the same. Not all component carriers aggregated by a user equipment may necessarily be Rel. 8/9 compatible. Existing mechanism (e.g. barring) may be used to avoid Rel-8/9 user equipments to camp on a component carrier.
A user equipment may simultaneously receive or transmit one or multiple component carriers (corresponding to multiple serving cells) depending on its capabilities. A LTE-A Rel. 10 user equipment with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple serving cells, whereas an LTE Rel. 8/9 user equipment can receive and transmit on a single serving cell only, provided that the structure of the component carrier follows the Rel. 8/9 specifications.
Carrier aggregation is supported for both contiguous and non-contiguous component carriers with each component carrier limited to a maximum of 110 Resource Blocks in the frequency domain using the 3GPP LTE (Release 8/9) numerology.
It is possible to configure a 3GPP LTE-A (Release 10) compatible user equipment to aggregate a different number of component carriers originating from the same eNodeB (base station) and of possibly different bandwidths in the uplink and the downlink. The number of downlink component carriers that can be configured depends on the downlink aggregation capability of the UE. Conversely, the number of uplink component carriers that can be configured depends on the uplink aggregation capability of the UE. It may not be possible to configure a mobile terminal with more uplink component carriers than downlink component carriers.
In a typical TDD deployment, the number of component carriers and the bandwidth of each component carrier in uplink and downlink is the same. Component carriers originating from the same eNodeB need not to provide the same coverage.
The spacing between centre frequencies of contiguously aggregated component carriers shall be a multiple of 300 kHz. This is in order to be compatible with the 100 kHz frequency raster of 3GPP LTE (Release 8/9) and at the same time preserve orthogonality of the subcarriers with 15 kHz spacing. Depending on the aggregation scenario, the n×300 kHz spacing can be facilitated by insertion of a low number of unused subcarriers between contiguous component carriers.
The nature of the aggregation of multiple carriers is only exposed up to the MAC layer. For both uplink and downlink there is one HARQ entity required in MAC for each aggregated component carrier. There is (in the absence of SU-MIMO for uplink) at most one transport block per component carrier. A transport block and its potential HARQ retransmissions need to be mapped on the same component carrier.
The Layer 2 structure with activated carrier aggregation is shown in FIG. 5 and FIG. 6 for the downlink and uplink respectively. The transport channels are described between MAC and Layer 1; the logical channels are described between MAC and RLC.
When carrier aggregation is configured, the mobile terminal only has one RRC connection with the network. At RRC connection establishment/re-establishment, one cell provides the security input (one ECGI, one PCI and one ARFCN) and the non-access stratum mobility information (e.g. TAI) similarly as in LTE Rel. 8/9. After RRC connection establishment/re-establishment; the component carrier corresponding to that cell is referred to as the downlink Primary Cell (PCell). There is always one and only one downlink PCell (DL PCell) and one uplink PCell (UL PCell) configured per user equipment in connected state. In the downlink, the carrier corresponding to the PCell is the Downlink Primary Component Carrier (DL PCC), while in the uplink it is the Uplink Primary Component Carrier (UL PCC).
Depending on UE capabilities, Secondary Cells (SCells) can be configured to form together with the PCell a set of serving cells. In the downlink, the carrier corresponding to an SCell is a Downlink Secondary Component Carrier (DL SCC), while in the uplink it is an Uplink Secondary Component Carrier (UL SCC).
The characteristics of the downlink and uplink PCell are:                For each SCell the usage of uplink resources by the UE in addition to the downlink ones is configurable (the number of DL SCCs configured is therefore always larger or equal to the number of UL SCCs, and no SCell can be configured for usage of uplink resources only)        The downlink PCell cannot be de-activated, unlike SCells        Re-establishment is triggered when the downlink PCell experiences Rayleigh fading (RLF), not when downlink SCells experience RLF        Non-access stratum information is taken from the downlink PCell        PCell can only be changed with handover procedure (i.e. with security key change and RACH procedure)        PCell is used for transmission of PUCCH        The uplink PCell is used for transmission of Layer 1 uplink control information        From a UE viewpoint, each uplink resource only belongs to one serving cell        
The configuration and reconfiguration of component carriers can be performed by RRC. Activation and deactivation is done via MAC control elements. At intra-LTE handover, RRC can also add, remove, or reconfigure SCells for usage in the target cell. When adding a new SCell, dedicated RRC signaling is used for sending the system information of the SCell, the information being necessary for transmission/reception (similarly as in Rel-8/9 for handover). In other words, while in connected mode, UEs need not acquire broadcast system information directly from the SCells.
When a user equipment is configured with carrier aggregation there is one pair of uplink and downlink component carriers that is always active. The downlink component carrier of that pair might be also referred to as ‘DL anchor carrier’. Same applies also for the uplink.
When carrier aggregation is configured, a user equipment may be scheduled over multiple component carriers simultaneously but at most one random access procedure shall be ongoing at any time. Cross-carrier scheduling allows the PDCCH of a component carrier to schedule resources on another component carrier. For this purpose a component carrier identification field is introduced in the respective DCI formats, called CIF.
A linking between uplink and downlink component carriers allows identifying the uplink component carrier for which the grant applies when there is no-cross-carrier scheduling. The linkage of downlink component carriers to uplink component carrier does not necessarily need to be one to one. In other words, more than one downlink component carrier can link to the same uplink component carrier. At the same time, a downlink component carrier can only link to one uplink component carrier.
Layer 1/Layer 2 (L1/L2) Control Signaling
In order to inform the scheduled users about their allocation status, transport format and other data-related information (e.g. HARQ information, transmit power control (TPC) commands), L1/L2 control signaling is transmitted on the downlink along with the data. L1/L2 control signaling is multiplexed with the downlink data in a subframe, assuming that the user allocation can change from subframe to subframe. It should be noted that user allocation might also be performed on a TTI (Transmission Time Interval) basis, where the TTI length is a multiple of the sub-frames. The TTI length may be fixed in a service area for all users, may be different for different users, or may even by dynamic for each user. Generally, the L1/2 control signaling need only be transmitted once per TTI.
The L1/L2 control signaling is transmitted on the Physical Downlink Control Channel (PDCCH). A PDCCH carries a message as a Downlink Control Information (DCI), which includes resource assignments and other control information for a mobile terminal or groups of UEs. In general, several PDCCHs can be transmitted in one subframe.
It should be noted that in 3GPP LTE, assignments for uplink data transmissions, also referred to as uplink scheduling grants or uplink resource assignments, are also transmitted on the PDCCH.
With respect to scheduling grants, the information sent on the L1/L2 control signaling may be separated into the following two categories, Shared Control Information (SCI) carrying Cat 1 information and Downlink Control Information (DCI) carrying Cat 2/3 information.
Shared Control Information (SCI) Carrying Cat 1 Information
The shared control information part of the L1/L2 control signaling contains information related to the resource allocation (indication). The shared control information typically contains the following information:                A user identity indicating the user(s) that is/are allocated the resources.        RB allocation information for indicating the resources (Resource Blocks (RBs)) on which a user(s) is/are allocated. The number of allocated resource blocks can be dynamic.        The duration of assignment (optional), if an assignment over multiple sub-frames (or TTIs) is possible.        
Depending on the setup of other channels and the setup of the Downlink Control Information (DCI)—see below—the shared control information may additionally contain information such as ACK/NACK for uplink transmission, uplink scheduling information, information on the DCI (resource, MCS, etc.).
Downlink Control Information (DCI) Carrying Cat 2/3 Information
The downlink control information part of the L1/L2 control signaling contains information related to the transmission format (Cat 2 information) of the data transmitted to a scheduled user indicated by the Cat 1 information. Moreover, in case of using (Hybrid) ARQ as a retransmission protocol, the Cat 2 information carries HARQ (Cat 3) information. The downlink control information needs only to be decoded by the user scheduled according to Cat 1. The downlink control information typically contains information on:                Cat 2 information: Modulation scheme, transport-block (payload) size or coding rate, MIMO (Multiple Input Multiple Output)-related information, etc. Either the transport-block (or payload size) or the code rate can be signaled. In any case these parameters can be calculated from each other by using the modulation scheme information and the resource information (number of allocated resource blocks)        Cat 3 information: HARQ related information, e.g. hybrid ARQ process number, redundancy version, retransmission sequence numberDownlink & Uplink Data Transmission        
Regarding downlink data transmission, L1/L2 control signaling is transmitted on a separate physical channel (PDCCH), along with the downlink packet data transmission. This L1/L2 control signaling typically contains information on:                The physical resource(s) on which the data is transmitted (e.g. subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA). This information allows the mobile terminal (receiver) to identify the resources on which the data is transmitted.        The Transport Format, which is used for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation (e.g. the number of resource blocks assigned to the user equipment)) allows the user equipment (receiver) to identify the information bit size, the modulation scheme and the code rate in order to start the demodulation, the de-rate-matching and the decoding process. The modulation scheme may be signaled explicitly.        Hybrid ARQ (HARQ) information:                    HARQ process number: Allows the user equipment to identify the hybrid ARQ process on which the data is mapped.            Sequence number or new data indicator (NDI): Allows the user equipment to identify if the transmission is a new packet or a retransmitted packet. If soft combining is implemented in the HARQ protocol, the sequence number or new data indicator together with the HARQ process number enables soft-combining of the transmissions for a PDU prior to decoding.            Redundancy and/or constellation version: Tells the user equipment, which hybrid ARQ redundancy version is used (required for de-rate-matching) and/or which modulation constellation version is used (required for demodulation).                        UE Identity (UE ID): Tells for which user equipment the L1/L2 control signaling is intended for. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other user equipments to read this information.        
To enable an uplink packet data transmission, L1/L2 control signaling is transmitted on the downlink (PDCCH) to tell the user equipment about the transmission details. This L1/L2 control signaling typically contains information on:                The physical resource(s) on which the user equipment should transmit the data (e.g. subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA).        The Transport Format, the user equipment should use for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation (e.g. the number of resource blocks assigned to the user equipment)) allows the user equipment (transmitter) to pick the information bit size, the modulation scheme and the code rate in order to start the modulation, the rate-matching and the encoding process. In some cases the modulation scheme maybe signaled explicitly.        Hybrid ARQ information:                    HARQ Process number: Tells the user equipment from which hybrid ARQ process it should pick the data.            Sequence number or new data indicator: Tells the user equipment to transmit a new packet or to retransmit a packet. If soft combining is implemented in the HARQ protocol, the sequence number or new data indicator together with the HARQ process number enables soft-combining of the transmissions for a protocol data unit (PDU) prior to decoding.            Redundancy and/or constellation version: Tells the user equipment, which hybrid ARQ redundancy version to use (required for rate-matching) and/or which modulation constellation version to use (required for modulation).                        UE Identity (UE ID): Tells which user equipment should transmit data. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other user equipments to read this information.        
There are several different possibilities how to exactly transmit the information pieces mentioned above in uplink and downlink data transmission. Moreover, in uplink and downlink, the L1/L2 control information may also contain additional information or may omit some of the information. For example:                HARQ process number may not be needed, i.e. is not signaled, in case of a synchronous HARQ protocol.        A redundancy and/or constellation version may not be needed, and thus not signaled, if Chase Combining is used (always the same redundancy and/or constellation version) or if the sequence of redundancy and/or constellation versions is pre-defined.        Power control information may be additionally included in the control signaling.        MIMO related control information, such as e.g. pre-coding, may be additionally included in the control signaling.        In case of multi-codeword MIMO transmission transport format and/or HARQ information for multiple code words may be included.        
For uplink resource assignments (on the Physical Uplink Shared Channel (PUSCH)) signaled on PDCCH in LTE, the L1/L2 control information does not contain a HARQ process number, since a synchronous HARQ protocol is employed for LTE uplink. The HARQ process to be used for an uplink transmission is given by the timing. Furthermore, it should be noted that the redundancy version (RV) information is jointly encoded with the transport format information, i.e. the RV info is embedded in the transport format (TF) field. The Transport Format (TF) respectively modulation and coding scheme (MCS) field has for example a size of 5 bits, which corresponds to 32 entries. 3 TF/MCS table entries are reserved for indicating redundancy versions (RVs) 1, 2 or 3. The remaining MCS table entries are used to signal the MCS level (TBS) implicitly indicating RV0. The size of the CRC field of the PDCCH is 16 bits.
For downlink assignments (PDSCH) signaled on PDCCH in LTE the Redundancy Version (RV) is signaled separately in a two-bit field. Furthermore the modulation order information is jointly encoded with the transport format information. Similar to the uplink case there is 5 bit MCS field signaled on PDCCH. 3 of the entries are reserved to signal an explicit modulation order, providing no Transport format (Transport block) info. For the remaining 29 entries modulation order and Transport block size info are signaled.
DRX (Discontinuous Reception)
DRX functionality can be configured for RRC_IDLE, in which case the UE uses either the specific or default DRX value (defaultPagingCycle); the default is broadcasted in the System Information, and can have values of 32, 64, 128 and 256 radio frames. If both specific and default values are available, the shorter value of the two is chosen by the UE. The UE needs to wake up for one paging occasion per DRX cycle, the paging occasion being one subframe.
DRX functionality can be also configured for an “RRC_CONNECTED” UE, so that it does not always need to monitor the downlink channels. In order to provide reasonable battery consumption of user equipment, 3GPP LTE (Release 8/9) as well as 3GPP LTE-A (Release 10) provides a concept of discontinuous reception (DRX). Technical Standard TS 36.321 Chapter 5.7 explains the DRX and is incorporated by reference herein.
The following parameters are available to define the DRX UE behavior; i.e. the On-Duration periods at which the mobile node is active, and the periods where the mobile node is in a DRX mode.                On duration: duration in downlink sub-frames that the user equipment, after waking up from DRX, receives and monitors the PDCCH. If the user equipment successfully decodes a PDCCH, the user equipment stays awake and starts the inactivity timer; [1-200 subframes; 16 steps: 1-6, 10-60, 80, 100, 200]        DRX inactivity timer: duration in downlink sub-frames that the user equipment waits to successfully decode a PDCCH, from the last successful decoding of a PDCCH; when the UE fails to decode a PDCCH during this period, it re-enters DRX. The user equipment shall restart the inactivity timer following a single successful decoding of a PDCCH for a first transmission only (i.e. not for retransmissions). [1-2560 subframes; 22 steps, 10 spares: 1-6, 8, 10-60, 80, 100-300, 500, 750, 1280, 1920, 2560]        DRX Retransmission timer: specifies the number of consecutive PDCCH subframes where a downlink retransmission is expected by the UE after the first available retransmission time. [1-33 subframes, 8 steps: 1, 2, 4, 6, 8, 16, 24, 33]        DRX short cycle: specifies the periodic repetition of the on duration followed by a possible period of inactivity for the short DRX cycle. This parameter is optional. [2-640 subframes; 16 steps: 2, 5, 8, 10, 16, 20, 32, 40, 64, 80, 128, 160, 256, 320, 512, 640]        DRX short cycle timer: specifies the number of consecutive subframes the UE follows the short DRX cycle after the DRX Inactivity Timer has expired. This parameter is optional. [1-16 subframes]        Long DRX Cycle Start offset: specifies the periodic repetition of the on duration followed by a possible period of inactivity for the DRX long cycle as well as an offset in subframes when on-duration starts (determined by formula defined in TS 36.321 section 5.7); [cycle length 10-2560 subframes; 16 steps: 10, 20, 30, 32, 40, 64, 80, 128, 160, 256, 320, 512, 640, 1024, 1280, 2048, 2560; offset is an integer between [0-subframe length of chosen cycle]]        
The total duration that the UE is awake is called “Active time”. The Active Time includes the on-duration of the DRX cycle, the time UE is performing continuous reception while the inactivity timer has not expired and the time UE is performing continuous reception while waiting for a downlink retransmission after one HRQ RTT. Similarly, for the uplink the UE is awake at the subframes where uplink retransmission grants can be received, i.e. every 8 ms after initial uplink transmission until maximum number of retransmissions is reached. Based on the above, the minimum active time is of length equal to on-duration, and the maximum is undefined (infinite).
The operation of DRX gives the mobile terminal the opportunity to deactivate the radio circuits repeatedly (according to the currently active DRX cycle) in order to save power. Whether the UE indeed remains in DRX (i.e. is not active) during the DRX period may be decided by the UE; for example, the UE usually performs inter-frequency measurements which cannot be conducted during the On-Duration, and thus need to be performed some other time, during the DRX opportunity of time, as exemplary illustrated in FIG. 5.
The parameterization of the DRX cycle involves a trade-off between battery saving and latency. For example, in case of a web browsing service, it is usually a waste of resources for a UE to continuously receive downlink channels while the user is reading a downloaded web page. On the one hand, a long DRX period is beneficial for lengthening the UE's battery life. On the other hand, a short DRX period is better for faster response when data transfer is resumed—for example when a user requests another web page.
To meet these conflicting requirements, two DRX cycles—a short cycle and a long cycle—can be configured for each UE; the short DRX cycle is optional, i.e. only the long DRX cycle is used. The transition between the short DRX cycle, the long DRX cycle and continuous reception is controlled either by a timer or by explicit commands from the eNodeB. In some sense, the short DRX cycle can be considered as a confirmation period in case a late packet arrives, before the UE enters the long DRX cycle. If data arrives at the eNodeB while the UE is in the short DRX cycle, the data is scheduled for transmission at the next on-duration time, and the UE then resumes continuous reception. On the other hand, if no data arrives at the eNodeB during the short DRX cycle, the UE enters the long DRX cycle, assuming that the packet activity is finished for the time being.
During the Active Time the UE monitors PDCCH, reports SRS (Sounding Reference Signal) as configured and reports CQI (Channel Quality Information)/PMI (Precoding Matrix Indicator)/RI (Rank Indicator)/PTI (Precoder Type Indication) on PUCCH. When UE is not in Active time, type-0-triggered SRS and CQI/PMI/RI/PTI on PUCCH may not be reported. If CQI masking is set up for the UE, the reporting of CQI/PMI/RI/PTI on PUCCH is limited to On Duration.
Available DRX values are controlled by the network and start from non-DRX up to x seconds. Value x may be as long as the paging DRX used in RRC_IDLE. Measurement requirements and reporting criteria can differ according to the length of the DRX interval, i.e. long DRX intervals may have more relaxed requirements (for more details see further below). When DRX is configured, periodic CQI reports can only be sent by the UE during “active-time”. RRC can further restrict periodic CQI reports so that they are only sent during the on-duration.
In FIG. 6 a per-subframe example of the DRX cycle is shown. The UE checks for scheduling messages (indicated by its C-RNTI, cell radio network temporary identity, on the PDCCH) during the “on duration” period, which is the same for the long DRX cycle and the short DRX cycle. When a scheduling message is received during an “on duration”, the UE starts an “inactivity timer” and monitors the PDCCH in every subframe while the Inactivity Timer is running. During this period, the UE can be regarded as being in a continuous reception mode. Whenever a scheduling message is received while the Inactivity Timer is running, the UE restarts the Inactivity Timer, and when it expires the UE moves into a short DRX cycle and starts a “short DRX cycle timer”. The short DRX cycle may also be initiated by means of a MAC Control Element. When the short DRX cycle timer expires, the UE moves into a long DRX cycle.
In addition to this DRX behaviour, a ‘HARQ Round Trip Time (RTT) timer’ is defined with the aim of allowing the UE to sleep during the HARQ RTT. When decoding of a downlink transport block for one HARQ process fails, the UE can assume that the next retransmission of the transport block will occur after at least ‘HARQ RTT’ subframes. While the HARQ RTT timer is running, the UE does not need to monitor the PDCCH. At the expiry of the HARQ RTT timer, the UE resumes reception of the PDCCH as normal.
There is only one DRX cycle per user equipment. All aggregated component carriers follow this DRX pattern.
In-Device Coexistence
For ubiquitous network access user equipments (UEs) are equipped with multiple radio transceivers, i.e. LTE, WiFi, and Bluetooth transceivers, and GNSS receivers. One resulting challenge lies in trying to avoid coexistence interference between those collocated radio transceivers. FIG. 7 shows an example of coexistence interference.
Due to extreme proximity of multiple radio transceivers within the same user equipment, the transmit power of one transmitter may be much higher than the received power level of another receiver. Given sufficient frequency separation and appropriate filters the transmit signal may not result in significant interference, but for some scenarios, e.g. different transceivers within the same user equipment operating on adjacent frequencies, current state-of-the-art filter technology might not provide sufficient protection against spurious emission. An illustration of an exemplary problem is shown in FIG. 8.
Coexistence Interference Scenario
Exemplarily shown here are coexistence interference scenarios between LTE radio and other radio technologies concerning the 3GPP frequency bands around the 2.4 GHz ISM band. The band layout is shown in FIG. 9.
LTE Coexisting with WiFi
There are 14 channels in the ISM band (from 2401 MHz to 2495 MHz) used for WiFi operation. Each channel has 5 MHz separation from the adjacent channels (with an exception of channel 14). The number of allowed channels for WiFi varies between countries (mostly 1 to 13 are allowed).
Within LTE band 40 the LTE transmitter will affect the WiFi receiver and vice-versa. As Band 7 is only used for UL communications in LTE, the WiFi receiver will be affected by LTE UL transmitter.
LTE Coexisting with Bluetooth
Bluetooth operates in 79 channels of 1 MHz each in the ISM band between 2402 MHz and 2480 MHz. Similar as WiFi case, the activities of LTE band 40 and BT will disturb each other, and the transmission of LTE in band 7 will affect the BT reception as well.
LTE Coexisting with GNSS
Examples of GNSS include GNSS systems (as GPS, Modernized GPS, Galileo, GLONASS, Space Based Augmentation Systems (SBAS), and Quasi Zenith Satellite System (QZSS)) operate in various frequencies globally with country specific deviations. The problematic cases for collocation of LTE and GNSS include Band 13 (UL: 777-787 MHz)/14 (UL: 788-798 MHz) which can cause interference to L1/E1 frequency of GNSS (1575.42 MHz) as it is close to the second harmonics of band 13/14 (1554-1574 MHz for band 13, 1576-1596 MHz for band 14), Galileo might support 2.5 GHz for GNSS, which will be affected by band 7 LTE and Indian Regional Navigation Satellite System uses IRNSS standard position and restricted services which are transmitted on L5 (1164-1215 MHz) and S (2483.5-2500 MHz) bands, which will be affected by band 7 LTE.
Solutions to In-Device Coexistence (IDC) Problems
Standardization efforts are ongoing in 3GPP, mostly in RAN Working Group 2, and are expected to be reflected in the standard with Release 11. The following section covers the agreements reached in above indicated working group.
The user equipment (UE) judges an LTE frequency as unusable when the ongoing IDC problem on this frequency between collocated LTE and ISM radio cannot be solved by the UE itself.
The assumption is that existing LTE measurements and/or UE internal coordination can be used as a baseline to trigger an indication of a In-device coexistence problem. What exactly triggers the indication is left to UE implementation. It is assumed that the network can trust the UE on the assessment of the IDC problem.
Upon triggering, the user equipment (UE) can send an indication to the network to report the IDC problem(s). It will not be specified when exactly the UE will send this indication. Only ongoing IDC interference on the serving or non-serving frequencies is indicated, this means assumptions or predictions of potential interference is not triggering an IDC problem.
In principal, an eNodeB may respond according to two possible approaches, namely by means of frequency division multiplexing or time division multiplexing reconfiguration of the UE affected by the IDC problem.
For the frequency division multiplexing (FDM) solution the UE will be re-allocated to a LTE frequency band not affected by the ISM interference. Concerning the time division multiplexing (TDM) solution, the LTE Rel-8/9/10 DRX mechanism is considered as a baseline to provide varying time division multiplexing (TDM) patterns (i.e. LTE scheduling and unscheduled periods) for resolving the IDC issues. A DRX based TDM solution should be used in a predictable way, i.e. the eNB should ensure a predictable pattern of unscheduled periods by means of the DRX mechanism.
All necessary/available assistant information for both frequency division multiplexing (FDM) and time division multiplexing (TDM) solutions is sent together to the eNodeB through the IDC indication. In case of an inter-eNB handover, the assistant information is transferred from the source to the target eNodeB.
The IDC indication is conveyed in a new UL-DCCH Message (i.e. RRC signaling) and can also be reused to send the updated assistant information, including the case that there is no longer an IDC problem.
The UE signaling for the IDC problem indication is shown in FIG. 10.
A user equipment (UE) can autonomously deny LTE transmission to protect ISM rare cases if other solutions cannot be used. It is assumed that the UE would deny ISM transmission in order to ensure connectivity with the eNodeB to perform necessary mechanisms to resolve IDC issues.
A prohibit mechanism is used to restrict the interval at which the UE may send IDC indications. It is for further study (FFS) whether the network indicates via dedicated signaling if the UE is allowed to trigger and send an IDC indication. It is also FFS whether the network indicates for which frequencies the UE may trigger an IDC indication and if so, how this information is provided.